This invention relates to cartridge fuse clips to be positioned in an electrical circuit and to releasably retain one end of a ferrule cartridge fuse positioned in the circuit. More specifically, the invention relates to a cartridge fuse clip which is capable of accepting and retaining the grooved end of one class of cartridge fuse and which is capable of rejecting the insertion of a cartridge fuse of any other class designation.
The structure of cartridge fuse clips is at least partially dictated by the structure of the cartridge fuse itself. Standards have been developed for governing fuses. These standards were first developed shortly after the turn of the century and incorporated as a part of the NATIONAL ELECTRIC CODE (NEC).
For approximately 50 years after the standard was adopted, however, it was not expanded to provide additional governance covering newer, more technologically sophisticated fuses. During the 1950's those in the industry became increasingly aware of the need for an expanded standard. Electrical associations and independant testing laboratories published tables of fuse dimensions and standards for low voltage fuses. The NEC dropped the table of fuse dimension from the 1959 issue of the code, and concentrated on safety requirements.
In 1959 the NEC included for the first time two new safety requirements regarding fuses. The two new requirements state that (1) fuse holders for current limiting fuses shall not permit insertion of fuses that are not current limiting, and (2) that the fuses shall be plainly marked with the ampere rating, voltage rating, interrupting rating where other than 10,000 amperes, "current limiting" were applicable, and the name or trademark of the manufacturer.
This requirement created a dilemma for the whole electrical industry. After months of committee studies and discussions, a solution was arrived at by forging a loophole in the requirement. The loophole solution simply eliminated the classification current limiting. A new classification was developed for high current interrupting fuses. While these fuses may also have current limiting characteristics, they can be used in standard fuse holders by merely marking them high current interrupting. Through this loophole, electrical manaufacturers have avoided the necessity of providing fuse clips capable of rejecting noncurrent limiting fuses.
Recently, the electrical associations and independent testing laboratories have begun to face up to the letter and scope of the requirements and have circulated proposed standards for a new class of fuses, identified as class R, which are current limiting with high current interrupting ratings. These fuses are the same size as the conventional fuses, such as the standard class H fuse, but are designed with an annular groove around one ferrule of the cartridge. Fuse clips for these class R fuses must accept the grooved end cap but must also reject the insertion of an identically sized conventional cartridge fuse without a groove in the end cap.